Our studies have been directed toward obtaining a greater understanding of the genetic control of protein structure and function using the ascomycete Neurospora crassa. Several systems are being examined: 1. The enzyme catalyzing the terminal step in tryptophan biosynthesis has been examined in a tryptophan synthetase mutant lacking normal activity. The purified enzyme appears to be a tetramer like the normal protein and a number of its physical and chemical properties have been described. 2. Cycloheximide-resistant mutants have been isolated and the mutations involved have been mapped. The ribosomes from a number of the mutants have been characterized. The basis for resistance seems to reside in structurally altered 80S ribosomes whose in vitro function (polynucleotide-directed amino acid incorporation) is also resistant to inhibition by cycloheximide. 3. Tryptophanyl-tRNA synthetase is being purified. Mutants with altered activating enzymes will be isolated for the purpose of evaluating the role of tryptophanyl tRNA on the regulation of the tryptophan biosynthetic enzymes. 4. Two systems capable of transporting glucose in N. crassa are being studied. Data so far indicate that the mechanism of transport is active transport rather than coupled transport or group translocation by the phosphotransferase system.